The invention relates generally to mixing and more particularly to a method and apparatus for mixing in which mixing containers holding the material to be mixed and the mixing impeller are releasably coupleable to a mixing drive station and in which the material is mixed in the container by rotation about the impeller about one axis of rotation while the entire container is rotated about another axis of rotation. This type of mixer is commonly referred to as a containerized batch mixer with multiple axes of rotation. Containerized batch mixers are especially useful for mixing particulate matter with or without the addition of liquids.
One known containerized batch mixer with multiple axes of rotation is disclosed in U.S. Pat. No. 4,468,129 to McIntosh et al. The mixer of McIntosh includes a cylindrical container with a built in mixing impeller and spear point, a docking station, upper and lower docking arms to move the container and engage the spear point with a drive mechanism, and multiple motors to rotate the impeller as well as the container. During the operation of this mixer, the container is loaded and closed prior to being mounted in the mixing station. The container is then lifted via the hydraulic docking arms to an operating position. In this operating position, the drive mechanism is engaged with the spear point of the impeller. The impeller is then rotated while the container itself is concurrently rotated.
A conventional containerized batch mixer 10 is illustrated in FIGS. 1A, 1B and 2. The mixer includes base 11 (which may be fixed to the ground), motor 12, rotational shaft 13, which rotates around horizontal rotation axis 14, docking assembly 15 and drive coupling assembly 20. The docking assembly 15 includes fixed upper docking arms 16, hydraulic lift cylinders 17, moveable lower docking arms 18, and container docking pads 30 (see FIG. 1B).
The drive coupling assembly 20 consists of drive motor 19, drive belt 21, drive shaft and spring-loaded collar 22 and drive socket 23. The drive socket 23 is designed to rotationally engage the drive end of the mixing container's impeller (spear point) about an initially vertical axis of rotation 29. The terminology "initially vertical" has been used to describe vertical axis 29 because once the docking assembly begins to rotate about the horizontal axis of rotation, vertical axis of rotation 29 also rotates about the horizontal axis of rotation.
The mixer 10 is shown in FIG. 2 with mixing container 24, which includes an impeller with mixing blades 25, a cylindrical skirt or false bottom 26, an impeller drive end or spear point 27 and an impeller shaft bearing assembly 28. The mixing container is shown in the loading position in FIG. 2. The container 24 is loaded onto the mixer 10 by rolling the container between lower docking arms 18 until the top of the container rests against container docking pad 30. The hydraulic lift cylinders 17 are then activated to move the lower docking arms 18 upward to engage the false bottom 26 of container 24. The hydraulic lift cylinders 17 then continue to raise the container 24 towards fixed upper docking arms 16. Impeller drive end or spear point 27 then enters drive socket 23 of drive coupling 20. When spear point 27 is fully engaged with drive socket 23, spring-loaded collar 22 of drive coupling 20 takes up further axial translation of the container and lower docking arms 18 until they reach the upper limit of their range of movement, identified as the operating position.
During operation, the docking assembly 15 is rotated about horizontal rotation axis 14 by motor 12 while the impeller with mixing blades 25 is rotated about the vertical axis of rotation 29 by drive motor 19.
The prior art containerized batch mixers described above work well and have been commercially successful, but suffer from several shortcomings. The mixers are relatively mechanically complex, and therefore costly to manufacture. The complexity arises from several sources. First is the lower docking arm which has a complex geometry and must be custom manufactured to fit the container utilized in the mixer. Second is the hydraulic drive system for the lower docking arm, which entails hydraulic pumps, tubing, and actuators and entails the risk of potentially contaminating leakages of hydraulic fluid. Further, the use of a hydraulic drive poses the risk that a sudden loss in power or hydraulic pressure would cause the lower docking arm to travel away from the fixed upper docking arm, which could allow the still rotating container to become separated from the mixing station. To address this risk, a backup, mechanical retention system, such as locking pins that fix the lower docking arm to the vertical support, are used. These locking pins must be custom located to fit each vessel's individual configuration. A third source of mechanical complexity and attendant cost is that the container must be formed with a cylindrical skirt, or false bottom, to provide a lower horizontal bearing surface by which the container can be supported by the lower docking arm. Fourth is the drive coupling, which is designed to accommodate axial misalignment and relative axial positioning of the drive socket and the container's spear point. The potential for axial misalignment arises from the imprecise positioning of the container on the lower docking arm and the lower docking arm relative to the drive coupling. The drive coupling is also designed to absorb relative axial movement of the spear point with respect to the drive motor as the container is brought into its fully raised position and after the spear point has engaged the drive socket of the drive coupling. The flexible, spring-loaded drive socket is mechanically complex and not as robust as could be desired to accommodate the increasing demands for mixing torque and power.
There is therefore a need to provide a mechanically simpler, more efficient, and less expensive containerized batch mixer.